An electronic device includes a multilayer substrate including a plurality of layers of conductors. The multilayer substrate has a first region and a second region divided in a planar direction of the multilayer substrate. The first region is provided with a main circuit that realizes a predetermined function. The second region is provided to have an additional circuit for adding a function to the predetermined function therein. The first region includes a first wiring for electrically connecting the main circuit to the additional circuit and a connecting portion disposed at an end of the first region adjacent to the second region and extending in the multilayer substrate to correspond to all of the conductors layered to electrically connect the first wiring to any of the conductors. The main circuit is connectable with the additional circuit through the first wiring and the connecting portion.